Some of it yes, but there is also a limit to how many people are willing to migrate. Mexico is only about 1/3 the size of the US and surveys indicate only 1/3

Some of it yes, but there is also a limit to how many people are willing to migrate. Mexico is only about 1/3 the size of the US and surveys indicate only 1/3 of Mexican adults have any desire to come to the US at all. Of those people, only half indicated willingness to come illegally and given that fact that they haven't done it yet, you can assume most of them won't. (This matters because there are limits on the number of legal immigrants) Assuming all Mexicans who are willing to actually make the journey here make it, and adding in immigrants from other Spanish speaking countries, it's still not enough to overcome the historical trend towards language assimilation.